Perfect And Real
In the perfect Scrum world, software is developed over time in fixed-size (ΔT) time boxes where all the work planned for the sprint is completed within the timebox. Typically ΔT is chosen to be 2 or 4 weeks.
If all the tasks allocated to a sprint during the planning meeting are accurately estimated (which never happens), and all the tasks are independent (which never happens), and any developer can perform any task at the same efficiency as any other developer (which never happens), then bingo – we have a perfect Scrum world.
However, in the real world (Scrum or non-Scrum), some (most?) tasks are interdependent, some tasks are underestimated, and some tasks are overestimated:
Thus, specifying a fixed ΔT timebox size for every single sprint throughout the effort may not be a wise decision. Or is it?
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Bulldozer00's Blog 
My experience is that many tasks are interdependent and many do not want to recognize or explore these interdependencies as it can be complicated and will require is to use our brain power and collaborate. The real world is hardly as simple as our models represent them…or is it?
My experience matches yours exactly. 🙂